Screen elements and screens of the above type are known, for instance, from EP-A-26 961 and US-RE 38,303 E which disclose different types of screens with screen elements arranged in a screen frame.
These screens have several advantages compared with those of earlier generations since individual screen elements can be exchanged when worn out, and since this exchange is a relative simple procedure which can be performed without requiring much work and equipment.
Since screen decks of the type above are subjected to longitudinal forces and also have screen elements arranged on a frame comprising longitudinally oriented bars, the screen elements must be locked somehow in the longitudinal direction to prevent sliding. A conventional way of achieving this is screwing into the side walls of the screen frame.
The screen elements should have an active surface that is as large as possible. The active surface is normally limited by the rigidity of the screen element since a screen element of lower strength requires supporting sections arranged at shorter intervals, which results in an increased dead surface of the screen element. Nevertheless, letting the entire screening surface consist of a single screen element and reducing the number of supporting points to a minimum does not constitute a convenient alternative. Such a method would certainly provide a maximum surface, but at the price of very high operating expenses since it would be necessary to exchange the entire screen deck also in case of local wear.
Consequently it is desirable to have a screen deck with a large active surface and high stability, on which it would be easy to exchange individual elements.
WO 89/08509 discloses what is referred to as a step deck, i.e. a screen where the successively arranged screen elements overlap each other to form a stepped screening surface.